Many mobile machines include DC electric traction motors for driving a propulsion device to propel the mobile machine. For example, locomotives often include multiple DC electric traction motors for driving wheels to propel the locomotive. Such a mobile machine may have its DC electric traction motors electrically connected in various manners. For example, at any given time, the DC electric traction motors of a locomotive are typically electrically connected all in parallel, all in series, or in a series-parallel configuration. In a series-parallel configuration, the locomotive may have multiple parallel connected branches of DC electric traction motors, each branch including a plurality of DC electric traction motors electrically connected in series.
When a DC electric traction motor drives a propulsion device, if the torque produced by the DC electric traction motor becomes greater than the resistance torque exerted against the DC electric motor by the propulsion device, the DC electric traction motor may accelerate. This may occur, for example, in the case of a DC electric traction motor driving a wheel of a locomotive if the wheel torque applied to the wheel by the DC electric traction motor becomes greater than the adhesion torque resulting from the adhesion of the wheel to the associated rail. In many circumstances, without knowing the precise value of the adhesion torque between the wheel and the rail, the operator of a locomotive may try to maximize acceleration by attempting to increase the wheel torque generated by the DC electric traction motor to a high percentage of the adhesion torque. In doing so, the operator may sometimes increase the wheel torque beyond the adhesion torque. Additionally, the adhesion torque between the wheel and the rail may abruptly decrease at some points on the rail for various reasons, which may also cause the wheel torque to exceed the adhesion torque. When the wheel torque exceeds the adhesion torque, the wheel may begin to slip on the rail. This may cause undesirable wear of both the wheel and the rail.
Additionally, when the wheel begins to slip on the rail the coefficient of friction between the wheel and the rail may change from a static coefficient of friction to a dynamic coefficient, which may significantly reduce the adhesion torque. This may compromise the acceleration of the locomotive compared to the rate of acceleration possible if the wheel maintains traction on the rail, thereby compromising the productivity of the locomotive. It may also cause the wheel to accelerate. The more rapidly the wheel accelerates after losing traction, the more time and corrective action it will take to regain adhesion between the wheel and the rail. When the wheel slips, the rate at which the wheel and the DC electric traction motor accelerate depends on the torque generated by the DC electric traction motor. The more rapidly the torque generated by the motor decreases, the less rapidly the electric motor will accelerate.
The torque generated by a DC electric traction motor depends in part on the net voltage across the DC electric traction motor, which equals the difference between the external voltage supplied to the DC electric traction motor and the magnitude of the opposing “back EMF” generated internally by the DC electric traction motor. If the external voltage remains constant, increasing the back EMF decreases the net voltage across the DC electric traction motor, thereby decreasing the current through the armature, which decreases the torque generated by the DC electric traction motor. The back EMF generated by a DC electric motor equals the product of the electric current in the field coil, the speed of the DC electric traction motor, and a constant. The positive correlation between the speed of the DC electric traction motor and the back EMF creates a tendency for the back EMF to increase with increasing speed.
However, the configuration of a typical “series-wound” DC electric traction motor produces an effect that partially offsets the positive correlation between the back EMF and speed. Generally, a DC electric traction motor includes a field coil (a stationary coil) and an armature (a rotating coil mounted on the rotor of the electric motor). A series-wound DC electric traction motor has its field coil and armature electrically connected in series, which tends to result in the field coil carrying the same magnitude of electric current as the armature. Because of this, any increase in the back EMF would cause a decrease in the electric current in both the armature and the field coil, which would have the effect of decreasing the back EMF. With the current in the field coil forced to decrease at the same rate as the current in the armature and thereby largely offseting the effect of the increased speed on the back EMF, the back EMF generated by a typical series DC electric traction motor increases somewhat gradually with increasing speed. Accordingly, when the mechanical load on a typical series-wound DC electric motor decreases abruptly, the motor may accelerate to a very high speed relatively quickly.
One strategy for correcting wheel slip is to simply decrease the electrical power supplied to the traction motors. However, due to operating characteristics typical of systems that supply electricity to the traction motors, the amount of electrical power supplied to the traction motors may decrease more slowly than desired.
Another alternative strategy is disclosed in U.S. Pat. No. 3,898,937 to Johnson (“the '937 patent”), which discloses a system for addressing wheel slip in a locomotive that has series-wound DC electric traction motors driving its wheels. The system of the '937 patent includes a DC generator that supplies electricity to a plurality of series-wound DC electric traction motors. The '937 patent discloses that the DC electric traction motors may be electrically connected to one another in parallel or in a series-parallel configuration. To reduce the torque output of a DC electric traction motor when wheel-slippage is detected, the system of the '937 patent uses an auxiliary current source to supply additional electricity to the field coil of the DC electric traction motor, thereby increasing back EMF and decreasing torque output. In the case of a series-parallel connection of the DC electric traction motors, when slip is detected in one of the branches of DC electric traction motors, the system uses auxiliary current sources to supply additional electricity to the field coils of each of the DC electric traction motors in that branch. This reduces the torque output of each of the DC electric traction motors in the branch, thereby counteracting the wheel slip.
Although the '937 patent discloses a system for counteracting wheel slip in a locomotive, certain disadvantages persist. For example, approach disclosed by the '937 patent for counteracting wheel slip when the DC electric traction motors are connected in a series-parallel configuration may unnecessarily decrease the torque output of some of the DC electric traction motors. When the locomotive operates with the DC electric traction motors connected in a series-parallel configuration, it can happen that wheel slip will occur for one of the DC electric traction motors in the branch while the wheels driven by the other DC electric traction motors in the branch may have good traction. When this occurs, the system of the '937 patent will unnecessarily reduce torque produced by the DC electric traction motors whose wheels have good traction, in addition to reducing torque output by the DC electric traction motor experiencing wheel slip.
The mobile machine and methods of the present disclosure solve one or more of the problems set forth above.